Laxatives are classified depending on their mechanisms of action. For example, laxatives comprising preparations of sodium phosphate or magnesium sulfate are of the osmotic type. Laxatives comprising preparations of phenolphthalein, bisacodyl, danthron, senna or cascara are of stimulant type and laxative comprising preparations of docusates, poloxamer, mineral oil or castor oil are of surfactant type. The osmotic, stimulant and surfactant laxatives have limited use due to their non-acceptable organoleptic properties and poor efficacy.
Yet another type of laxative is bulk laxative which on oral administration swells significantly occupying most of the intestinal lumen, thereby improving gastrointestinal motility to cure constipation. Bulk laxative is also used in the treatment of diarrhoea. Bulk laxative may comprise semisynthetic polymers such as calcium polycarbophil or methyl cellulose or natural products such as bran, psyllium or isphagula husk. Due to the low swellability of calcium polycarbophil ie calcium substituted polymer formed of acrylic acid monomer and a crosslinker under physiological conditions, it is required to be prescribed in high doses of 4–6 g/day (ie 8–12 tablets of 500 mg/day). Methyl cellulose also has limited use due to its high dosage requirement (6 gm/day). Amongst the bulk laxatives, isphagula husk is most widely used. It exhibits low swelling (˜40 times) in simulated intestinal fluid (USP 23 Page No 2053) and necessitates administration of large doses (7 gm/day) which causes discomfort and results in loss of appetite. Besides swelling in the intestine, isphagula husk also significantly swells in the stomach occupying a large volume and causes abdominal pain and discomfort. Proteins associated with the natural product isphagula husk are known to cause sensitisation of the mucous membrane, irritation and other allergic reactions in the body. Isphagula husk has low dispersibility in water because of which it forms lumps or agglomerates in aqueous fluids. The lumps swell non-uniformly and adversly affect gastrointestinal motility. Moreover, being a natural product, isphagula husk is prone to microbial contamination during storage.
U.S. Pat. No. 4,777,232 describes such polymers comprising water soluble monomers such as acrylic acid or methacrylic acid and a polysaccharide such as starch or derivative thereof. It is formed in combination with a surface active agent and employs the polysaccharide in an amount of 10–70% by weight of the monomer. U.S. Pat. No. 3,997,484 discloses a graft polymer formed of polyacrylonitrile and gelatinised starch as polysaccharide. Graft polymers of acrylic monomers such as acrylonitrile with polysaccharide such as starch or cellulose derivative are known (U.S. Pat. Nos. 4,076,663, 4,931,497 and 5,011,892). U.S. Pat. No. 5,340,853 describes a mixture of 20–98%, preferably 10–50% by weight of a polymer such as polyacrylic acid, polymethacrylic acid, polyacrylamide or polymethacrylamide and 2–80%, preferably 10–50% by weight of a polysaccharide such as tragacanth or guar gum, gum arabic, starch, dextran, cellulose or derivative thereof. Polymers comprising polysaccharides are reported to find applications in diapers, sanitary napkins, tampons, surgical pads and sheets, paper towels, electrolyte thickeners in dry batteries, moisture conserving materials in agriculture or drying agents. The polymers of the above US Patents are not known or reported to have been used as laxative. Due to gelatinisation of starch, the polymers will show low swellability (30 to 40 times) in physiological fluids though their swellability in water is high. The starch in such graft polymers is likely to undergo digestion in the physiological fluid, and reduce the swellability of the graft polymers. Moreover such polymers are not slimy. High percentages of polysaccharides in polymer have been found to reduce the swellability thereof in physiological fluids.
Crosslinked, water soluble, water absorbable or water swellable polymers formed of monomers such as acrylates, acrylic acids, acrylamides, acrylonitriles or vinyl pyrrolidones and cross linkers are known and reported to be used as absorbing or dehydrating or thickening agents in sanitary napkins, diapers, shaving system or for bioadhesion (U.S. Pat. Nos. 5,985,944, 4,070,348, 5,354,290, 5,716,707, 5,804,605, 4,618,631, 5,514,754, 5,626,154, 4,535,098, 5,973,014, 5,221,722 and 4,267,103; EP Patent No 105634B1 and JP Patent No 6322178A).
Gums such as tragacanth, acacia or xanthan gum are water swellable and are known to be used as emulsifiers, binders, stabilisers, thickening or suspending agents in food and pharmaceutical industry.